It deals with methods of sending secret messages ranging from
pencil and paper systems performed by hand to today's advanced block
ciphers, such as Rijndael, the cipher chosen
as the Advanced Encryption Standard, as depicted at right.

Introduction

This page is about codes and ciphers, which people use
to communicate with each other in ways that other parties
cannot (it is hoped) understand. Although secrecy in communication
can precede literacy, for example by the use of obscure
allusions, a spoken language that is different from the
one commonly spoken, a jargon or cant of terms with special
or secondary meanings, or a conventionalized way of speaking
such as Pig Latin, the efflorescence of many and
sophisticated methods of secret communications waited for
the development of alphabetic writing, which allows any thought
to be represented by a small number of easily manipulated
characters.

Even then, it took a conceptual breakthrough to realize that
letters can be represented by other symbols;
particularly in introductory books on the subject for
children, this is often illustrated by various examples that are
used today, such as Morse code,

signal flags,

Baudot,

ASCII,

Braille,

and semaphore:

And, for another even more prosaic illustration, here is the color
code used on resistors (as well as on the old mica capacitors) and the
color code, if one can call it that, of pool balls:

Of course, today, cryptography has extended into the realm of advanced
mathematics, including number theory, which is concerned with prime numbers.

One early and entertaining historical survey of the use of codes
and ciphers was the book Secret and Urgent, by Fletcher
Pratt, also the author of several novels. This book came out in the
same year as Elementary Cryptanalysis, by Helen
Fouché Gaines, which will be mentioned below. The title of
this book is a particularly apt description of why methods of
scrambling messages to keep them secret are used.

Obviously, if a message contains nothing that is confidential,
there is no need to bother putting it into code or cipher.

But equally, if a message is not urgent, then even if it is
secret, it can be communicated at some time when there is an opportunity
to meet privately.

Only when both conditions exist: when the contents of a message
must be kept secret, and yet the message is so urgent that the risk
must be taken of sending it by a means that may allow others to read it,
would people take the time and effort to put a message into cipher,
and take the risk involved in relying on cipher to maintain its
secrecy.

Of course, today computers carry out the steps involved in enormously
complicated cipher schemes at the push of a button, so neither the
effort nor the risk looms as large as it did during most of the history
of the subject.

This site contains a brief outline of the various types of cipher
systems that have been used historically, and tries to relate them
to each other while avoiding a lot of mathematics.

Suggestions for Further Reading

The following books can be recommended for someone beginning to learn about
the subject of secret writing:

The Codebreakers, David Kahn,
Macmillan (1st ed.) Simon and Schuster (2nd ed.).
This book is a fascinating history of cryptography,
dealing with the role it has played in many historical events.
There are also some nuggets of technical
information not seen in other books aimed
at the general public, and there is
historical information about subjects related to
secret codes, such as cable codes (which do not involve secrecy,
and were for saving money on sending telegrams).

Elementary Cryptanalysis, Helen
Fouché Gaines, Dover. Published under
the title Cryptanalysis to avoid
confusion with a book by Abraham Sinkov (also a
good book), this book deals with pencil and paper
ciphers, and is particularly aimed
at people who solve such ciphers as a hobby.
It describes a wide variety of ciphers
and a multitude of solving methods.

And I will also mention two other books here:

Machine Cryptography and Modern Cryptanalysis, Cipher A.
Deavours and Louis Kruh; Artech House. This book is a gold mine of information
and was the source for much of what you will see in this web site about
cipher machines of the rotor era. I had believed this book to be out of print,
but in fact it is still available through Artech House, which has selected it
as one of those works sufficiently valuable that it will be reprinted should
their stocks run out, as part of its "In Print Forever" program. Unfortunately,
it was marred by a number of typographical errors. I had thought that
regrettable, but felt that this was a common occurrence in books with
a limited anticipated sale. An otherwise positive
review in Cryptologia magazine (also a significant
source, particularly for my account of the Siemens T-52) did include
the comment that one might expect better than that, and as a result
my faulty memory led me to categorize the review as "scathing", for
which I apologize to its author (himself a significant source of
information for my section on the Enigma's Uhr box), Frode Weierud.
A number of the illustrations from this book
were reprinted (with full permission, of course) in the more recent
book Decrypted Secrets from Springer-Verlag.

Applied Cryptography, Bruce Schneier; John Wiley and Sons.
This book is aimed at the computer professional who needs to
implement secure systems involving cryptography. As it describes
a wide selection of algorithms and protocols, it will be of interest
to anyone concerned with cryptography in the computer era. This
book is one of the most widely available books covering the subject
matter, and it is frequently cited as a source and as an authority
on the USENET newsgroup sci.crypt. The 2nd edition is considerably
expanded, with fascinating detail on a much larger number of
cipher systems.

Notes on the Purpose of this Page

This page is about cryptography, it does not fall into
certain categories of worthwhile and helpful pages about cryptography that
are more common; it is neither:

a page introducing beginners to methods of solving different kinds of
paper and pencil ciphers,

a page explaining how you can obtain a copy of PGP, ScramDisk, or
Private Idaho to start protecting your own communications, or

a page devoted to the history of cipher machines, with photographs of
various ones.

There are links to some of the pages in these categories in the
Links
section of this site.

Occasionally, some methods of cryptanalysis are briefly touched upon
here, but the details are very limited, compared to the excellent material
available elsewhere.

This site has a great deal in common with sites of the third category,
but alas, it doesn't include any photographs. What it does have are
schematic diagrams (in my own, somewhat nonstandard symbolism, designed to
be easy to recognize at small sizes) and descriptions of the operation of
many historical cipher machines. One such schematic diagram is the one
below of the workings of the Enigma:

The story of the Enigma's decryption,
derived from a multitude of secondary sources, is, I hope, explained
with both completeness and clarity here.

It covers forms of cryptography ranging from the simple
paper-and-pencil methods to the modern computer cipher systems, and
attempts to point out the common features that link them.

It also deals with other related topics, such as protocols for secure
communications, such as Kerberos:

One word of warning, however: I have indulged my own ego rather
shamelessly here, and have described a series of block ciphers of my own
design (under the name of "Quadibloc";
the first one was inspired by DES
and Blowfish, although in a way it was the opposite of Blowfish, and the
others are the result of appropriating various ideas found in the AES
candidate ciphers), some paper-and-pencil fancies of mine, and a rather
elaborate fractionation scheme for converting the binary output of modern
encryption methods to letters for transmission by Morse, or base-78 armor
(more efficient than base-64, if less efficient than base-85), or
encryption by classical letter-based methods.

In only one section do I discuss, and very briefly, codes, in which
words or phrases rather than letters, bits, or digits are the unit of
encipherment. However, the word code is used legitimately in
mathematics to refer to substitutions which are non-linguistic (and hence,
in cryptology, would be called ciphers) from Morse code to Hamming code
(used for error-correction) and Huffman code (used for data compression).
I have, therefore, been unable to be rigorous about the use of the word
"code" in these pages.